differences between XBEE1 and Xbee2

What are the main differences between XBEE1 and Xbee2, how can I replace a Xbee1 with a Xbee2?
If I just replace it (the Xbee1 with the Xbee2), it doesn’t works!
I am using it just in transparent mode.

Differences between S1 and S2 modules can be found at the following links, http://www.digi.com/support/kbase/kbaseresultdetl?id=2213
and http://www.digi.com/technology/rf-tips/2007/05